Upper airway patency is presumed to be maintained by neuromuscular mechanisms. In conditions such as obstructive sleep apnea and during diaphragm pacing, airway occlusion appears to occur in the region of the oropharynx but is associated with ineffective muscle activation at all levels of the upper airway. Basic information is lacking on the mechanical properties of upper airway structures in order to understand where the upper airway is vulnerable to collapse and how muscles might act to modulate resistance. The proposed studies to be performed in anesthetized dogs, are designed to evaluate these issues. Pressure-flow relationships in the nasal, oropharyngeal, and laryngeal regions will be evaluated under conditions of bias flow through a tracheal cannula. Regional geometry and compliance will be determined from measurements of cross-sectional area using biplane cine-radiography and area-distance estimations using sound wave reflection techniques. Measurements of regional resistance and compliance will be performed before and after respiratory stimulation with pharmacologic agents, electrical stimulation of various upper airway muscles, and physical manipulation of upper airway structures. These techniques also will be used to evaluate the site and mechanism of obstruction during diaphragm pacing in animals spontaneously breathing through the upper airway. To alleviate obstruction, we will stimulate electrically upper airway muscles. We will directly alter the timing and amplitude of upper airway and diaphragm stimulation to determine how changes in the relative activation of upper airway and chest wall muscles affect flow resistance, lung volume, and ventilation. We have proposed a model of control of respiration based on our observed patterns of activation of the genioglossal muscle and diaphragm. Results from these studies will be used to expand this model to account for the mechanical effects of upper airway muscle activation. This theory may give insight into mechanisms responsible for periodic breathing and sleep apnea. Such information may be helpful in developing a more rational approach to the diagnosis and treatment of obstructive and central apneas in humans.